There is a trend amongst commercially available granular detergents towards higher bulk densities and towards granular detergent components which have a higher content of detergent active ingredients.
Many of the prior art attempts to move in this direction have met with problems of poor solubility properties arising from low rate of dissolution or the formation of gels. A consequence of this in a typical washing process can be poor dispensing of the product, either from the dispensing drawer of a washing machine, or from a dosing device placed with the laundry inside the machine. This poor dispensing is often caused by gelling of particles which have high levels of surfactant upon contact with water. The gel prevents a proportion of the detergent powder from being solubilised in the wash water which reduces the effectiveness of the powder. Another adverse consequence arises even if the powder is well dispensed and dispersed in the washing water if it does not dissolve rapidly. The wash cycle has a limited duration during which the detergent can act upon the laundry. If the cleaning action is delayed because the powder is slow to dissolve, this, too, will limit the effectiveness of the powder.
The process engineer and formulator have frequently found that the need for good dispensing and the need for good dissolution rate have placed conflicting demands upon them. The solution has generally been to find a compromise which gives adequate dispensing and adequate dissolution rate. For example, poor dispensing of high bulk density granular detergents is often associated with surfactant rich particles having a high specific surface area, either due to high porosity or a small particle size (especially "fines"). However, decreasing the porosity and/or increasing the average particle size cause the dissolution rate to decrease.
Prior art has dealt with the problems of processing granules in a number of different ways which are intended to optimise the physical properties of the resulting powders.
One processing route is described in GB 2 166 452, published on May 8th, 1986. This patent proposes a process for making a detergent composition with a high bulk density and good solubility properties. This process is based on agglomeration in a kneader forming pellets, followed by granulation to the required particle size distribution in, for example, a hammer mill.
Other processes in the prior art have sought to avoid the step of granulating pellets which is capital-intensive and expensive. One such process is a high speed mixer into which a liquid binder is sprayed, coupled with a lower speed mixer/agglomerator to form the final physical properties of the granules, including particle size and shape. It is apparent from the prior art that choice of starting materials, choice of liquid binder, and choice of processing parameters can give a wide range of finished products.
U.S. Pat. No. 4,846,409, published on Jul. 11th, 1989, proposes a process of granulating a powder in succession in a high speed mixer and a low speed mixing granulator, followed by drying in a fluidised bed into which a binder is sprayed. The process is said to be particularly suitable for dyestuff granules. Water is the preferred granulating liquid.
EPA 367 339, published on May 9th, 1990, also proposes a sequence of processing steps comprising a high speed mixer/densifier, a moderate speed granulator/densifier and a drying or cooling step. The process is aimed primarily at densification of porous spray-dried particles by deforming them in order to produce a finished particle with a porosity of less than 10%, and preferably less than 5%.
WO 92/04398, published on 19th Mar., 1992, describes a process for making agglomerates by extrusion followed by granulation. The resulting surfactant rich particles have a low porosity and contain very few or no fines. This results in good dispensing properties, but slower dissolution rate.
European application 92200993.1 assigned to the Applicants describes a granulation process which uses a high active surfactant paste as the binder. This application is a part of the state of the art in accordance with Article 54 (3) EPC.
The present invention provides a continuous process for making a high bulk density granular composition with improved solubility properties, in particular, a composition which has both good dispensing and good dissolution rate. The process for making this composition can produce surfactant rich particles with a porosity which is favourable for a good rate of dissolution (about 10% or higher), but which is still well below the porosity of spray dried powder (up to 70%) which have a much lower bulk density. The process of the invention also produces surfactant rich particles with a narrow particle size distribution which contains few "fines" and which favours good dispensing.
This is achieved by the choice of starting materials, especially choice of the pre-neutralised surfactant paste which acts as the liquid binder, which are essentially different from the starting materials described in the prior art. Furthermore, this process is characterised by a short residence times in the mixing and granulating units when the process is operated according to the present invention.